This invention relates in general to data collection systems and, more particularly, to status indicators for telephone line powered telemetry devices.
During the span of the last decade, the technical feasibility of employing telemetry devices to collect data from remote sites has proven to be a viable technology. The continuing development of this fledgling industry has resulted in the development of telemetry devices which are dedicated to flawless, highly reliable performance at very low costs. This new class of telemetry device is used in automatic data collection and remote monitoring (ADCARM) systems.
Like most viable systems, automatic data collection and remote monitoring (ADCARM) systems prosper by providing vital services which cannot be economically duplicated by any other means. ADCARM systems are employed most advantageously in automating certain types of repetitious tasks. Since the switched public telephone communications system reaches almost every home in the United States, there is a natural symbiosis between ADCARM systems and that telephone system to share the resources of the already existing system infrastructure.
Moreover, to encourage cooperation with the telephone company, ADCARM service providers offer risk free revenue incentives to the telephone company by designing telemetry equipment which transparently overlays the existing telephone network, without modification and without any degradation in subscriber telephone service. Hence, when completely installed, some ADCARM systems require only access to the "test trunk" (an existing test port on the central office switch) while other configurations require absolutely no access to the central office facilities whatsoever. Consequently, in order capitalize upon the significant economic advantages gained by sharing the service resources of an already existing infrastructure, successful ADCARM service providers develop low cost telemetry devices which cannot interfere with the operation of the public telephone network. Compatibility is assured by designing subservient telemetry devices which relinquish control and automatically disengage themselves under all contention scenarios where a subscriber demands telephone system resources.
There are basically two types of ADCARM systems, called "dial inbound" and "dial outbound". The "dial outbound" system is most frequently employed in automatic meter reading (AMR) systems to collect utility meter readings from customer premises while the "dial inbound" systems are typically used with copy or vending machine monitors, in addition to AMR applications. The designations "inbound" and "outbound" refer to the method necessary to initiate a telemetry exchange with respect to the central control mechanism for the data system. Thus a "dial outbound" system requires a central data controller (CDC) unit to handshake with (ie. dial out to) a remotely located telemetry device, to trigger the telemetry exchange. Conversely, a "dial inbound" system is one which, according to our definition, "dials in" to the CDC unit under its own volition without requiring any initiating handshake. Most often, "outbound" telemetry units can be called at will since the CDC unit essentially "wakes" them up while "inbound" units, being self-activating, are only available for telemetry exchanges when they "call in" at a designated time. Dial inbound telemetry devices often incorporate real time clocks which can be reprogrammed by the CDC unit when they "dial in" at the appointed time. Despite these operational differences at the system level, the remotely located telemetry devices have very similar operating requirements.
Dial outbound telemetry devices are activated by sending an alerting signal to them while the subscriber telephone line is not being used. Since this signal is designed not to ring the telephone set, the user is unaware of the telemetry transaction. The telemetry device, however, upon reception of this alerting signal (typically a tone burst of specified frequency), seizes the phone line and completes the telemetry exchange. Access is made through the test trunk, itself part of the central office switch, to facilitate the transmission of this alerting signal to the outbound telemetry device without ringing the subscriber telephone. Therefore the procedure to collect telemetry data from the dial outbound telemetry unit is very straightforward but does require access to the test trunk at the central office facility. Because dial outbound telemetry devices incorporate an information signal detector (such as an alert signal detector) which is always anticipating the reception of an alerting signal, they are well suited to applications which require random collection of telemetry data since they can be called at will.
Because dial outbound devices require access to the test trunk, dial outbound telemetry systems are best suited to very large ADCARM systems which have a significant population of telemetry devices, at each central office switch, to justify the economic and political commitment. Just such an arrangement exists for utility companies since nearly every residence which has a telephone set almost certainly will purchase other metered commodities such as water, gas, and electricity. In fact, this arrangement is so natural that this form of dial outbound telemetry device has spawned an entire industry dedicated to automatically collecting utility meter readings. However, access to the test trunk, essential to the operation of dial outbound systems, also requires approval from the telephone company. If this cooperation is not forthcoming, dial inbound systems must be used.
Because dial inbound telemetry devices simply dial-in through the conventional telephone network to a host computer at a preprogrammed time, they require no access to any of the infrastructure of the telephone system itself. Like dial outbound systems, dial inbound systems are utilized in automatic meter reading (AMR) applications to collect utility meter readings from customer premises at a predetermined time. Additionally, dial inbound telemetry is well suited for applications which require routine or periodic telemetry exchanges. Some degree of flexibility is afforded to dial inbound units because the next "call-in" time for the telemetry device can be downloaded from the host as part of the telemetry exchange. Still, even with the ability to dynamically select the next "check-in" time, dial inbound devices are not well suited to applications requiring the random collection of telemetry data, because the device is inaccessible until it self-activates. It should be noted that dial inbound telemetry devices are essentially asleep until they are activated by input from a real time clock, or alarm input, whereas dial outbound units are always anticipating the reception of an alerting signal.
From the telephone subscriber's or user's perspective, both dial inbound telemetry devices and dial outbound telemetry devices are simply other devices which plug into the subscriber's telephone jack (in addition to the subscriber telephone set). Therefore such telemetry devices must not interfere with the operation of the telephone system. Consequently, while the subscriber's telephone set is in use, the dial inbound unit cannot be allowed to go off-hook whereby it would attempt to dial over an ongoing phone call. Moreover, it is also not desirable for the device to become active while the ring signal, intended to "ring" the subscriber telephone set, is present. Were the device to become active during such a ring signal, the device would erroneously "answer" an inbound phone call.
Because dial inbound telemetry devices only become active at a designated time (some can be activated by an alarm signal applied to special input terminals), it is preferable to view them from a slightly different perspective than dial outbound systems. Nevertheless, both dial inbound and outbound telemetry devices have similar requirements with respect to checking the status of the telephone line to which they are coupled. Stated alternatively, before a dial inbound telemetry unit becomes active in response to an activating signal from a real time clock (or an alarm signal), it must first ascertain the status of the subscriber telephone line. If that line is currently being activated by a ring signal or is currently in use by the subscriber, the dial inbound unit must wait until the line is again on-hook before it can begin its "dial in" procedure. ADCARM telemetry devices incorporate "line status indicator circuits" to provide these required supervisory functions.
Conventional line status indicators have employed a plurality of separate circuits to provide an indication of the line status. For example, such separate and distinct circuits within a prior line status indicator have included a high voltage detector responsive to the 90 volt AC ring signal, a "static off-hook detector" which would prevent the telemetry device from becoming active while the subscriber telephone set was in use, and a "dynamic off-hook detector" which would disengage the telemetry device should the telephone set be taken off-hook during a telemetry transmission. The terms "static" and "dynamic" refer to the state of the telemetry device (quiescent or active, respectively) when determining the status of the subscriber telephone line. For example, in an outbound telemetry device, the quiescent state or mode refers to state of the device as it awaits an alerting signal whereas the active state or mode refers to the state of the device once it has received an alerting signal which activates the device to transmit the data collected thereby.
Ring detector devices of the prior art often employ a separate AC coupled (capacitively coupled) circuit in conjunction with an optocoupler responsive to the high voltage 90 volt AC ring signal to detect that ring. For example, many contemporary answering machines use this approach. By selecting the appropriate coupling capacitance, an appropriate high AC voltage ring threshold voltage can be set.
At the present time, the most common application for ADCARM systems, both dial inbound and dial outbound, is in automatic meter reading (AMR) applications. The following is a brief discussion of the configuration of one conventional AMR system which demonstrates the basic structure and the elements of such a system. FIG. 1 shows a simplified block diagram of several residences 10, factories 15 and businesses 20 coupled via trunk lines 25 to a central office (CO) 30. At first glance, the operation of AMR systems may seem deceptively straightforward. However, the pragmatic aspects of designing the individual components within the system has been a major obstacle hindering the implementation of AMR systems. Of course, any AMR equipment placed at customer premises to be highly reliable, cost effective, and must not interfere with normal subscriber telephone service. It is only very recently that equipment capable of meeting the stringent requirements of this technology has become available.
As seen in FIG. 1, a typical AMR system uses the same telephone lines which provide normal subscriber telephone voice service without any alteration of telephone company equipment. When an AMR system is present on a subscriber's telephone line, there is no perceivable difference to the customer as to how the voice telephone system operates in comparison to an identical telephone system without AMR capability. In AMR systems, it is very desirable to have a minimal impact on the design of the existing telephone network.
FIG. 2 is a block diagram of the additional equipment required at the customer's premises (10, 15 or 20) to make the operation of the AMR system possible. To be non-intrusive with respect to the voice operation of the subscriber's phone line, the AMR equipment at the subscriber's premises simply "bridges" the existing telephone circuits. If properly designed, the AMR equipment will not negatively affect the operation of that telephone equipment.
As seen in FIG. 2, an MIU (meter interface unit) 35 is connected in parallel with the subscriber telephone line 25 at each remote site or customer premises. Connected in a similar parallel manner to the phone line are the telephone set 40 and other devices 45 which the customer might use such as answering machines, FAX telecopiers, computer modems and the like. For purposes of this discussion, one user device will not be distinguished from another and, in this context, a "telephone set" is used to mean any one of the user supplied devices.
It is again emphasized that the MIU connects to or bridges the phone line without adversely effecting the operation of the other devices on the line and that this property is not an inherent feature of the telephone network. While the user supplied devices (phone, fax, modem, etc.) are under the direct control of the consumer who provides them, the MIU is part of a network belonging to a utility company or utility meter agency which needs to collect utility use data. Since the MIU and the telephone set cannot function simultaneously on the same subscriber line, one or the other must have priority. Since the AMR system is automated and the telephone company will not tolerate any degradation in subscriber telephone service, the choice, by default, is that the user must have priority over any AMR function.
Attached to MIU 35 are one or more electronic registers 50 which are physically attached to the bodies of the utility meters 55. These registers can be read electronically by the MIU but may also have the same dials as their mechanical counterparts. These registers 50 serve to electronically collect the amount of metered commodity delivered to a customer, just as mechanical registers record such information mechanically with indicating dials. In most AMR systems, the electronic register converts the mechanical motion of a flow sensor into a serial format, similar to an RS-232 format, which can be electronically transferred when the device is interrogated in a prescribed manner. This minimizes the number of wires required to electrically interface the MIU to the electronic register. It is desireable, although not required, that the MIU be powered directly from the phone line without any reliance on external power sources. Dial inbound MIU's, however, can almost be totally line powered, if desired, except for a small battery required to avoid a power interruption of the real time clock which is an integral part of the design of an inbound MIU.
As seen in the block diagram of central office 30 in FIG. 3, a single central office site serves a plurality of remote users. Central office 30 includes a central office "switch" 60 having a plurality of ports. Phone lines 25 coming into the central office are essentially a bundle of wires which are connected to one or more of punch down blocks 65 at central office 30. From punch down blocks 65, the subscriber line pairs within the bundle are connected to central office switch 60. Each subscriber line has a known port on central office switch 60 which can be uniquely addressed by calling a specific telephone number.
As previously noted, only dial outbound telemetry devices require access to any hardware located at the central office facility. Remotely located telemetry devices of the dial inbound variety can be looked at as automatic telephones which just dial through the switched public telephone network into a host computer.
Associated with the central office switch 60 is a test trunk 67 which the telephone company uses to test subscriber lines attached to the switch 60. These tests help the service provider to ascertain the condition of any or all of the cable pairs attached to the switch, for maintenance purposes. Although most subscribers are unaware of such a function, the telephone company routinely checks the condition of the telephone line on a regular basis. Since these testing capabilities are an integral part of the central office switch design, the test trunk provides the required interface for a dial outbound AMR controller 70. By using the test trunk, dial outbound AMR controller 70 utilizes capabilities already incorporated into the switch by design, to minimize disruptions to subscriber service caused by activity on the test trunk. Thus test trunk 67 is the access point for dial outbound AMR controller 70 to selectively connect to a given MIU on a particular subscriber's phone line. Part of the AMR function is to maintain a table of "phone" numbers which can be "dialed" on the test trunk thereby providing connectivity to the desired MIU device or devices.